1. Field of the Invention
This invention relates to epitaxy of high temperature superconductive materials on silicon, and more particularly to methods and structures wherein high temperature copper-oxide superconductors are epitaxially formed on a substrate including a (001) silicon surface.
2. Description of the Prior Art
Recently, the remarkable discovery by J. G. Bednorz and K. A. Mueller, reported in Z. Phys. B.-Condensed Matter 64, 189 (1986) and Europhysics letters, 3,379 (1987) completely changed the direction and importance of superconducting technology. Their discovery was that certain metallic oxides can exhibit superconducting transition temperatures considerably in excess of 23K. These materials are often termed "High T.sub.c Superconductors". Since the initial discoveries of Bednorz and Mueller, a vast amount of research and development has been undertaken around the world to further study these types of superconducting materials in order to extend even farther the temperature range over which the materials are superconducting, as well as to understand the basic mechanisms for superconductivity in this class of materials.
Bednorz and Mueller first showed superconducting behavior in mixed copper-oxides, typically including rare earth and/or rare earth-like elements and alkaline earth elements, for example La, Ba, Sr, . . . , and having a perovskite-like structure. Materials including the so called "1-2-3" phase in the Y-Ba-Cu-O system have been found to exhibit a superconducting transition temperature in excess of 77K.
R. B. Laibowitz and co-workers were the first to achieve and describe a method for making thin films of these materials. These thin film structures and methods for making them are described in co-pending application Ser. No. 436,851 filed Oct. 31, 1989 which is a divisional application of Ser. No. 356,565 filed May 22, 1989 also co-pending which is a continuation of Ser. No. 027584, filed Mar. 18, 1987 now abandoned and assigned to the present assignee. The work of Laibowitz et al. is also described in Phys. Rev. B, 35, 8821 (1987). In this technique, a vapor transport process is used in which the components of the superconducting film are vaporized and deposited on a substrate in an oxygen atmosphere, after which the deposited film is further annealed.
Another paper describing thin films of these high T.sub.c superconductors, and specifically high critical currents in these materials is P. Chaudhari et al, Phys. Rev. Lett., 58, 2684, June 1987. Chaudhari et al. described epitaxial high T.sub.c superconducting films formed on substrates such as SrTiO.sub.3, in which the critical current at 77K was in excess 10.sup.5 A/cm.sup.2.
Other references generally describing the deposition of films or layers of high T.sub.c superconducting materials include J. Cuomo, co-pending application Ser. No. 276,085 filed Nov. 23, 1988 which is a continuation of Ser. No. 043,523, filed Apr. 28, 1987, now abandoned and A. Gupta, co-pending application Ser. No. 121,982, filed Nov. 18, 1987 now abandoned, and also assigned to the present assignee. The first of these co-pending applications describes a plasma spray coating process while the second describes a method for coating a substrate, as by spraying from solution, and then patterning the coated film to eventually produce a patterned layer of high T.sub.c superconducting material.
Epitaxy of high T.sub.c superconducting films has been accomplished on several substrates, including SrTiO.sub.3. In particular, superconducting films capable of carrying high critical currents have been epitaxially deposited as noted in a paper by P. Chaudhari et al, published in Physical Review Letters, 58, 2684, June 1987. However, while the epitaxial films have been prepared on substrates having a close lattice match to that of the superconducting film, no one has provided successful epitaxy on a substrate, such as silicon, whose lattice does not match well to that of the high T.sub.c superconducting film. Further, because silicon based materials have been shown to react with high T.sub.c superconducting films in a manner to destroy the superconductivity of these films, the provision of an epitaxial film which retains its superconducting properties at high temperatures without also destroying the desirable properties of silicon, would be a significant accomplishment.
Accordingly, it is a primary object of the present invention to provide a method for epitaxially depositing high T.sub.c superconducting films on a substrate including silicon, and the film-substrate combination thereby produced.
It is another object of the present invention to provide epitaxy of high T.sub.c superconducting films on silicon in a manner in which the high temperature processing required to produce the superconducting properties in these films does not adversely affect the superconducting films and the silicon-including substrate.
It is another object of the present invention to provide epitaxy of high T.sub.c superconducting films on a substrate including silicon, where the film-substrate combination is tolerant of high temperature processing steps.
It is another object of the present invention to provide high T.sub.c superconducting films/silicon-based epitaxial combinations which can be produced by many procedures as long as proper crystallographic orientations are maintained between the substrate and the superconducting film.